The processing of solids using supercritical fluids is typically carried out by either batch or semi-continuous methods. In a production environment, these methods require an increased number of vessels, as well as large volumes. Additionally, batch processes are more time consuming and carry a higher risk of product contamination. There is also an increase in equipment fatigue due to pressurization and depressurization steps and supercritical fluid losses during the process. Because of these factors, supercritical fluid processing has been directed toward specialty products instead of commodity products. These disadvantages, however, may be overcome by the development of a continuous process for near-critical or supercritical fluids.
Supercritical fluids have gained wide acceptance in the past decade for their use in a number of processes. They are unique in the sense that density can be manipulated by simply changing pressure or temperature. Thus, all density-dependent properties are also varied. This makes supercritical fluids ideal candidates for extraction solvents. At a given set of conditions, a substance can be dissolved and extracted in the supercritical fluid. Once extracted, the extracted product can be separated from the supercritical fluid simply by modifying the thermodynamic properties of the fluid (changing temperature and/or pressure) or by a separating device such as an absorption column or an adsorption vessel. No further separation steps are necessary. Carbon dioxide is a popular supercritical fluid because it is nontoxic, inexpensive, and widely available. Another popular fluid that can be used is propane. Propane is also relatively inexpensive and can be used at low pressures.
A wide variety of solids can be processed using supercritical fluids such as various vegetable seeds, food ingredients, herbs, botanicals, solids contaminated with toxic organic material or pharmaceutical materials. By performing the extraction with a supercritical fluid, as opposed to an organic solvent, such as hexane, residual solvent levels are negligible and meet regulatory standards for both the food and pharmaceutical industries. Therefore, by developing a continuous process for solids that utilizes supercritical fluid technology, many advantages can be realized.
Some continuous processes for solids have been developed in the past, including U.S. Pat. No. 4,675,133 and U.S. Pat. No. 5,041,245. While these patents disclose continuous processes for the extraction of vegetable oil, a screw press was used in both instances to transfer the material into the vessel. While screw feeders are widely known in the art, they do possess certain limitations. In particular, a screw press typically imparts force upon the material and compacts it while moving it from one place to another. Extracting from a highly-compacted material can reduce yield and quality because the reduced surface area does not allow for intimate contact between the starting material and the extracting fluid. Additionally, a screw press has pressure limitations and requires a significant amount of energy input.